<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Title</title>
  <script type="text/javascript">

		// let num = undefined;
		// if (typeof(num) ==  'undefined') {
		// 	console.log("num是undefined");
		// } else {
		// 	console.log("num不是undefined");
		// }


  </script>
</head>
<body>

</body>
</html>
